The present invention relates to an integrated circuit capable to operate without contact by means of at least one coil forming a tuned resonant circuit with a tuning capacity, and comprising a charge pump with two clock inputs.
The integrated circuits used to implement chip cards, electronic labels and in a general way integrated circuits mounted on portable supports comprise generally an EEPROM memory (electrically erasable and programmable memory) to record and store data, as well as a booster circuit to produce a high voltage for programming or erasing said memory. As a matter of fact, a programming or erasing voltage of an EEPROM memory is typically about 15 to 20 V, when the supply voltage Vcc of an integrated circuit is about 3 to 5 V only.
In the field of microelectronics, the preferred embodiment of a booster circuit is a charge pump, which is easy to integrate on silicon.
The problem when using a charge pump is however that it must be driven by clock signals. Such signals must be provided by an oscillator, which generally consumes some current. In the case of a contactless integrated circuit supplied by electromagnetic induction and having low energetic resources, such a current consumption may be not desirable.
Before dealing with this technical problem in more details, the conventional structure of a charge pump and the conventional arrangement of such a charge pump in a contactless integrated circuit will be recalled.
The charge pump shown in FIG. 1 comprises a plurality of capacities arranged in cascade, for example N capacities C.sub.1 to C.sub.N. The anode of each capacity C.sub.1, C.sub.2. . . is coupled to the anode of the following capacity C.sub.2, C.sub.3, . . . by means of MOS transistors T.sub.1 to T.sub.N having their gate fed back to their drain and equivalent to diodes. At the end of the chain, the transistor T.sub.N Couples the anode of the capacity C.sub.N to the anode of a storing capacity C.sub.hv whose cathode is connected to ground. The cathodes of the odd numbered capacities C.sub.1, C.sub.3 . . . are driven by a clock signal Hi applied to one input E.sub.1 of the charge pump and the cathodes of the even numbered capacities C.sub.2, C.sub.4, . . . are driven by a signal H.sub.2 applied to one input E.sub.2, the signal H.sub.2 having its phase opposite with respect to the signal H.sub.1. Thus, alternately, each odd numbered capacity C.sub.1, C.sub.3, . . . discharges into the following even numbered capacity C.sub.2, C.sub.4 . . ., and each even numbered capacity C.sub.2, C.sub.4, . . . discharges into the following odd numbered capacity C.sub.3, C.sub.5, . . . At the end of the chain, the capacity C.sub.N discharges into the capacity C.sub.hv whose terminals, present a high voltage V.sub.hv.
FIG. 2 represents a conventional arrangement of the charge pump 10 within a contactless integrated circuit 20. The integrated circuit 20 comprises a coil L forming a tuned resonant circuit LCa with a tuning capacity Ca, allowing the integrated circuit to receive an alternating voltage Va by electromagnetic induction. The charge pump 10 is connected by its clock inputs E.sub.1 and E.sub.2 to an oscillator 15 which is controlled by a signal PGR and produces the clock signals H.sub.1 and H.sub.2. The oscillator 15 receives a supply voltage Vcc from a rectifier bridge Pd using diodes or transistors, which receives the induced alternating voltage Va at its input and comprises a filtering capacity Cf at its output. A conventional embodiment of the oscillator 15 is shown in FIG. 3. Three cascading inverting gates I.sub.1, I.sub.2, I.sub.3 are arranged in closed loop by means of an AND gate referenced A1 and controlled by the signal PGR. The signal H.sub.1 is for example taken at the output of the last gate I.sub.3 and the inverse signal H.sub.2 provided by a fourth inverting gate I.sub.4.
The tuning capacity Ca is generally an adjustable capacity, adjusted so that the resonance frequency of the circuit LCa is as close as possible to the oscillating frequency of the magnetic field in which the integrated circuit 20 is intended to work. As shown, the tuning capacity Ca for example comprises several capacities Ca.sub.1, to Ca.sub.n in parallel, the metallic tracks enabling the connection of some capacities having been cut at the time of adjusting.
Thus, when an erasing or writing operation of an EEPROM memory (not shown) has to be performed, the signal PGR is set to 1, the gate A1 becomes transparent, the oscillator 15 starts and the charge pump 10 is activated.
As mentioned above, the working of the oscillator 15 implies a non-negligible current consumption, due to the fast commutation of the various inverting gates. At the start of an erasing or programming operation, when the signal PGR is set to 1, this consumption is added to the consumption of the charge pump 10 which has to perform the charge of the storing capacity NChv. Furthermore, in a contactless chip card or an electronic label, such an erasing or programming operation can be started when the reception conditions of the induced voltage Va are bad. Thus, if the energy received by the coil L is too week, the supply voltage Vcc may drop, causing the end of the working of the integrated circuit.
It is thus desirable, in a contactless integrated circuit, to reduce as much as possible the current consumption during the periods when the high voltage Vhv is generated.
In the state of the art, there is also known a method consisting in directly activating a charge pump by means of the positive and negative half waves of an alternating voltage induced in a coil.
This method, illustrated in FIG. 4, consists in connecting the two terminals of the coil L to the two inputs E.sub.1 and E.sub.2 of the charge pump by means of two switches 16, 17 controlled by the programming signal PGR. When the signal PGR is at 1, the switches 16, 17 are closed and the half waves Va1 and Va2 are directly sent to the charge pump 10 as the activation signals H.sub.1 and H.sub.2.
However, the applicant has remarked that this method, although allowing the suppression of the oscillator 15, has the drawback of detuning the resonant circuit LCa.
As a matter of fact, referring to the diagram of FIG. 1, a charge pump considered from its inputs E.sub.1 and E.sub.2 is equivalent to a capacity C.sub.E with a value EQU C.sub.E =N C/2 (2)
N being the number of stages of the charge pump and C the value of the capacities C.sub.1, C.sub.2, . . . C.sub.N of each stage.
Therefore, in FIG. 4, when the signal PGR switches to 1 and the charge pump 10 is so connected to the coil L, the capacity C.sub.E substantially detunes the resonant circuit LCa and the energy reception is done in bad conditions.
The aim of the present invention is to reduce this drawback.
U.S. Pat. No. 5,206,495 describes a chip card with two operating modes, contact or contactless, comprising an integrated circuit, a contact field to operate in the contact mode, and two coils to operate in the contactless mode.
U.S. Pat. No. 5,285,370 describes a device wherein the voltage that is induced on the terminals of a coil is used to activate the clock inputs of a charge pump. However, the device is provided with a "wide band" inductive coil without tuning capacitor, which does not form a resonant circuit. Moreover, this document recommends to dispose a switch between the coil and the clock inputs of the charge pump, so that the coil is connected to the charge pump only when necessary.